Watches wherein the crystal is secured to the middle part by means of a bezel are already known. For example, U.S. Pat. No. 3,688,492 discloses and shows a watch including a flat crystal which is secured to the middle part by driving a bezel into a groove formed in the top face of the middle part. The bezel includes an inner shoulder which abuts axially against the top radial face of a radial collar of the crystal to hold it pressed against the middle part.
With this type of watch, when an axial shock is applied to the crystal, for example, when it falls from the height of a table, stresses are mainly applied axially because of the flat external face of the crystal. Stresses can thus be taken up axially by the middle part at the surface of the crystal that axially abuts the middle part. Thus, the crystal works mainly in axial compression, and it does not undergo any bending.
This is not the case when the watch is provided with a crystal of convex hemispheric shape. Indeed, the axial shock is then applied on the central part of the crystal, which causes a significant deformation of the crystal, which tends to bend and collapse in its central part, while the collar of the crystal tends to move outwards.
These deformations of the crystal can cause cracks, or even breaks. Such cracks can be due in particular to the central part of the crystal coming into contact with the to end of the arbour of the display hands of the watch or to internal stresses in the material forming the crystal.
In the aforecited document, if axial stress localised in the central part of the crystal caused an outward radial movement of the collar, the crystal would then abut against the top end section of the bezel, which would apply torsional stress that is detrimental to its mechanical resistance. Moreover, since the bezel is driven in towards the inside of the case via its internal edge, the torsional stress would be applied in the direction that the bezel is disassembled, with a significant risk of disassembling the crystal.
It is an object of the invention to eliminate these drawbacks in a simple and economical manner.
For the purpose, the invention proposes a watch of the type previously described, characterized in that the collar includes a convex external peripheral surface which is generally complementary to a concave inner surface of the bezel arranged opposite, underneath the radial abutting surface of the bezel, and which abuts against said inner surface radially if an axial shock is applied to the crystal.
Owing to the arrangement according to the invention, in the event of an axial shock, the flexion stresses are decreased which decreases the risk of cracks. The crystal of the watch according to the invention thus has improved shock resistance.
Moreover, it is generally necessary to provide pre-centring means on the crystal collar in order to position the crystal properly with respect to the middle part, prior to assembling the bezel; otherwise there is a risk of the crystal being damaged by the bezel, or it being impossible to assemble the bezel. These pre-centring means can be the cause of premature breakage of the crystal, because of the significant deformation caused by an axial shock.
In order to overcome this problem, according to an advantageous feature of the invention, on the side of the inner peripheral edge of the collar, the bottom radial face of the collar includes at least one positioning lug which extends downwards, between the dial and the inner axial wall of the middle part, and the radial distance between the lug and the inner axial wall of the middle part is greater than the radial distance between the outer surface of the collar and the inner surface of the bezel, such that, in the event of an axial shock, the outer surface of the collar abuts radially against the inner surface of the bezel before the lug comes into contact with the inner axial wall of the middle part.
According to another feature of the invention, the outer surface of the collar and the inner surface of the bezel have complementary frustro-conical forms.
These forms enable optimum shock resistance to be obtained.
According to another feature of the invention, the middle part includes an outer peripheral edge comprising a substantially frustro-conical concave inner wall having a downwardly increasing diameter, the bezel includes an annular heel portion, which is driven axially, from the top downwards, into the middle part, such that the outer peripheral wall of the heel portion is pressed radially against the inner wall of the edge, and in that the inner surface of the bezel is formed by the inner wall of the heel portion.
This feature enables the middle part to bear the stresses applied by the crystal to the inner surface of the bezel.
According to another feature of the invention, the outer wall of the heel portion has a frustro-conical shape which defines an angle smaller than the angle defined by the inner surface of the bezel.
This feature means that the bezel can be driven into the middle part without any risk of overloading the crystal.